What should my spring rate be?
Choosing the right spring rate depends on what you’re suspending (bike, car, or motorcycle), your weight and gear, and your riding or driving goals. A practical starting point is to relate spring rate to the load on the spring and the amount of deflection you want under that load, using the basic relation k = W/δ, where k is the spring rate, W is the load, and δ is the deflection. From there, you tailor the rate to your specific setup and test ride feedback.
How to determine the correct spring rate for your setup
Use this method to estimate a target spring rate from your weight, travel, and desired sag. It’s a solid starting point for coil springs and can guide adjustments for air springs or progressive designs.
- Calculate the total load on the spring. For a bicycle, this is rider weight plus gear; for a car, it’s occupant and cargo distribution; for a motorcycle, rider plus luggage. Use kilograms or newtons consistently (1 kg ≈ 9.81 N).
- Decide on your target sag (deflection) as a fraction of total travel. A common target is about 25–30% of available travel for performance setups, with somewhat less sag for comfort-focused configurations. Translation: δ = travel × sag_fraction.
- Compute the needed spring rate with k = W/δ. If you know W and δ, you can estimate a starting rate. Example: a 800 N load with a 0.03 m deflection (30 mm) gives k ≈ 26,700 N/m, or about 26–27 N/mm.
- Account for geometry and load transfer. Front vs rear in cars, or fork vs shock in bikes, can shift how much load each spring bears. Use separate calculations for each spring if you have independent circuits.
- Test and refine. Install an initial spring rate, measure sag with the rider on board (or with full load in the car), and adjust up or down in steps. If you bottom out, go stiffer; if you never reach target sag, go softer or adjust preload/air pressure accordingly.
The above steps give you a practical starting point. Real-world results depend on your exact vehicle geometry, the type of spring (coil, air, or progressive), and how you tune preload, dampers, and ride height. After an initial setup, verify sag with an actual load and make iterative refinements.
Application-specific guidelines
Bicycles
For bicycles, especially mountain bikes with coil springs (or when sizing a coil kit for a fork or rear shock), rider weight and terrain dictate the spring choice. Air-sprung forks and shocks change feel with pressure, so use the same sag-based approach while accounting for how air pressure affects effective rate. Use these guidelines to start, then fine-tune on the trail.
- Estimate rider weight plus typical gear or backpack if you ride with a pack.
- Set target sag around 25–30% of total fork or shock travel with the rider aboard.
- Calculate a starting k with k = W/δ, using W and δ from your bike’s travel. Use this as a baseline even if your system is air-sprung; pressure adjustments will modify the effective rate.
- Begin with OEM or manufacturer-recommended spring kits when available, then adjust preload and, if applicable, air pressure to approach the target sag.
Testing on varied terrain is essential. If you hit bottom too easily, go stiffer or increase preload; if you feel too unsettled or line is harsh, go softer or reduce preload while keeping the sag target in mind.
Cars
Car springs are chosen to achieve a balance between ride comfort and handling, while maintaining the desired ride height. If you’re upgrading to performance springs or adjusting factory springs, start from the OEM setup and adjust in small increments. Remember that load distribution (front vs rear) and the overall suspension design influence the ideal rate.
- Determine anticipated loads on each axle (occupants, cargo, and any residual loads). Heavier loads may require stiffer front or rear springs to preserve balance.
- Define a target ride height and acceptable ride quality. Sport setups often tolerate a stiffer rate but must avoid bottoming on rough pavement.
- When upgrading, consider compatibility with dampers and alignment. A stiffer rate without matching damping can worsen ride quality or handling.
- Use manufacturer recommendations as a baseline; if you measure sag or ride height changes, adjust the rate or preload accordingly to maintain proper geometry.
After installation, test the car on typical routes and trackable maneuvers (corners, bumps, highway sections) to verify that the suspension maintains control without excessive body roll or harshness. Fine-tuning may involve damper adjustments or slight rate changes rather than large jumps.
Motorcycles
Motorcycle suspensions rely on spring rate to support rider weight and luggage while delivering predictable handling. A typical approach is to aim for a sensible amount of sag with the rider seated, then adjust for comfort or sport intent.
- Calculate rider weight with gear, then estimate front and rear loads separately based on typical riding posture and weight transfer.
- Aim for about 25–30% sag of total travel with the rider aboard; adjust if you carry luggage or lean into corners differently.
- Compute an initial rate using k = W/δ for each end (front and rear). If you’re using progressive springs, plan for an initial rate, then expect effective rate to rise as the spring compresses.
- Test ride at varied speeds and on rough surfaces; consider upgrading to progressive springs or adding spacers if bottoming occurs or if you want more mid-stroke support.
Motorcycle setups often require additional attention to damping and rebound in combination with spring rate. Dial-in with a skilled suspension tuner if you’re chasing precise handling or extreme terrain conditions.
Summary
There is no single “one-size-fits-all” spring rate. The right rate emerges from the load on the spring, the desired deflection (sag), and the suspension’s geometry and goals. Start with W/δ calculations based on your weight and travel, then verify with real-world testing and adjust preload, air pressure, or the spring itself as needed. Whether you ride a bike, drive a car, or pilot a motorcycle, grounding your choice in sag-based measurements and incremental testing will guide you toward a setup that feels balanced, comfortable, and controlled.
What is 20k spring rate?
KG/mm to LB/in
| Spring Rate (kg/mm) | Spring Rate (lb/in) |
|---|---|
| 17k | 952 lb/in |
| 18k | 1008 lb/in |
| 20k | 1064 lb/in |
| 22k | 1120 lb/in |
What does 7K spring rate mean?
That translates to 7.1kg/mm, usually known simply as a “7K spring.” In Newtons-per-millimeter, it's 70N/mm. Going up in spring rate — increasing the spring's stiffness — means moving from 400 lb/in, up to 450 lb/in, for example. For softer springs, it's the other direction.
Is a higher spring rate better?
Higher spring rates will give you a firmer feel, and many racecar drivers tend to prefer stiffer springs to reduce body roll and body lean. They also want low ground clearance to maintain a low center of gravity.
What is the ideal spring rate?
Springs should typically be compressed 25-30% of the free length when supporting the weight of the vehicle. Drag race cars will normally use a lighter rate spring (about 30%) to promote weight transfer while a street car will use a firmer rate spring (about 25%).
